Conventionally, an inkjet printhead (to be referred to as a printhead hereinafter) is known which causes a heater arranged in each nozzle of the printhead to generate thermal energy, makes ink near the heater bubble by using the thermal energy, and discharges the ink from the nozzles by bubble to execute printing.
Recent inkjet printing apparatuses using the printhead are required to have high printing speed and high resolution. To meet this requirement, many nozzles are implemented in the printhead at a high density. As for driving of the heaters in the printhead, there is a demand for driving as many heaters as possible simultaneously at high speed from the viewpoint of printing speed.
Normally, a number of heaters and their driving circuit are formed on a single semiconductor substrate (this substrate will be referred to as a head substrate hereinafter). For this reason, the heater driving circuit is formed by using a MOS semiconductor process that is capable of inexpensively forming small devices at high density in a simple manufacturing process as compared to a conventional bipolar semiconductor process.
A method of driving a heater by a predetermined current has been proposed in Japanese Patent Publication Laid-Open Nos. 2004-181678 and 2004-181679 as a new heater driving method coping with the high-speed printing and MOS manufacturing process.
FIG. 18 is a block diagram showing the arrangement of a printhead heater driving circuit according to Japanese Patent Publication Laid-Open No. 2004-181679.
As is apparent from FIG. 18, the heater driving circuit comprises a reference voltage circuit 105, voltage-to-current conversion circuit 104, and current source block 106. The current source block 106 includes m heater groups each accommodating x heaters. One printhead comprises n current source blocks. Hence, one printhead comprises a total of (x×m×n) heaters.
The reference voltage circuit 105 generates a reference voltage (Vref) as the reference of the voltage-to-current conversion circuit 104. The voltage-to-current conversion circuit 104 converts a voltage to a current on the basis of the reference voltage (Vref) from the reference voltage circuit 105, i.e., generates a reference current (Iref) from the reference voltage (Vref).
On the basis of the reference current (Iref) generated by the voltage-to-current conversion circuit 104, a reference current circuit (not shown) generates a plurality of reference currents proportional to the reference current (Iref). The reference currents are supplied to the n current source blocks.
In each of the n current source blocks, on the basis of reference currents IR1 to IRn, current sources 1031 to 103M output constant currents Ih1 to Ihm proportional to the reference currents supplied to the current sources.
As shown in FIG. 18, the current source block 106 comprises the (x×m) heaters, switching elements 102 as many as the heaters, and the constant current sources 1031 to 103m corresponding to the m groups. The short and open states of the current between the terminals of each switching element 102 are controlled by a control signal from a control circuit of the printing apparatus main body. Each of m groups accommodates x heaters 101 and x switching elements 102. In the groups, heater resistances 10111 to 101mx and switching elements 10211 to 102mx for driving and controlling the heater resistances are connected in series. In the groups, power-supply-side terminals are commonly connected to a power supply line 110, and ground-side terminals are commonly connected to a GND line 111 through the constant current sources.
The output terminals of the constant current sources 1031 to 103m provided for the m groups 106-1 to 106-m, respectively, are connected to the common connection terminal of the groups 106-1 to 106-m in which the heaters 101 and switching elements 102 are connected in series. Current driving control of the heaters is executed by turning on/off the switching elements 102 in the groups by a control signal. The output currents Ih1 to Ihm from the constant current sources 1031 to 103m provided for the groups are supplied to desired heaters.
In an actual printhead, a plurality of (n) current source blocks 106 having the same structure are provided. The heater driving operation in each current source block 106 is the same as described above. When the same operation is executed in the n current source blocks 106, any desired heaters of the (x×m×n) heaters are driven and generate heat.
In recent inkjet printing apparatuses, to further improve the quality of printed images, the gamut is extended by using inks of many colors, or the size of ink droplets is reduced for high-resolution printing. These lead to an increase in number of nozzles that discharge inks in the printhead or an increase in number of nozzle arrays.
As described above, when heaters are driven by the constant current method, as described above, in a head substrate on which a plurality of heater arrays are arranged, the constant current source blocks to supply a predetermined current to the heaters must be arranged in arrays corresponding to the heater arrays. Hence, the number of reference currents to supply reference currents to the constant current sources increases.
When the number of reference currents increases, current consumption of the whole head substrate increases. Heat generated by the current consumption raises the temperature of the head substrate.
When the temperature of the head substrate rises, the temperature of ink that contacts the head substrate rises. This increase in temperature makes ink discharge unstable or degrades the print quality.